The present invention relates to a semiconductor integrated circuit device and, more particularly, to a semiconductor integrated circuit device in which data stored therein internally are required to be held even after a main power voltage is cut off.
In a semiconductor integrated circuit device having a data storage circuit composed of volatile memories such as RAM's, it is frequently required to hold an internal conditions such as data thereof as they are even after a main power supply voltage thereto is removed. For this purpose, it is well-known in the art to use a backup power source for applying a backup voltage to the device which the main power voltage is being removed.
Specifically, as shown in FIG. 5, a main power source 60 which provides a main power or normal operating voltage Vcc is connected through a power switch 61 and a diode 62 to one power terminal 51 of an integrated circuit device (IC) 50 and a backup power source 65 is also connected to the terminal 51 through a diode 66. The other power terminal 52 of IC 50 is grounded (GND). Since a backup voltage V.sub.BUP is lower than the normal operating voltage Vcc, when the power switch 61 is turned on to cause IC 50 to perform a normal operation, the diode 66 is reverse-biased to disconnect the backup power source 65 from IC 50. On the other hand, when the switch 61 is turned off to disconnect the main power source 65, the diode 66 becomes conductive and thus the backup voltage V.sub.BUP is applied to the power terminal 51. IC 50 is thus brought into a backup condition. The diode 62 is turned off at this time. Thus, the internal states of the integrated circuit device 50 can be held as they are even after the main power source is disconnected.
In such construction as above, however, it is required to provide the diode 66 for disconnecting the backup power source 65 from the power terminal 51 in the normal operation state of IC 50. The increase in number of parts to be mounted externally on the integrated circuit device 50 not only makes a system using IC 50 expensive, but reduces the production yield thereof. Moreover, the backup voltage V.sub.BUP is in fact applied to the power terminal 51 with a voltage drop corresponding to the forward voltage drop V.sub.F of the diode 66. For this reason, in order to hold the internal states of IC 50 in the backup condition, it is necessary for the backup power source to generate an actual backup voltage V.sub.BUP which is higher than the minimum voltage necessary to hold the internal states of the device at least by the voltage V.sub.F. A battery is used as the backup power source 65 in general. The fact that the relatively high backup voltage is required to the battery means that a life time thereof is shortened correspondingly.